Method for producing deep-drawing low-carbon steel sheet



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- r 1 3,404,047 METHOD FOR PRODUCING DEEP-DRAWING LOW-CARBON STEEL SHEETI Stephen R. Goodman, Monroeville, andHsun Hu, Franklin Township,Westmoreland County, Pa., assignors to United States Steel Corporation,a corporation of Delaware.

No Drawing. Filed Dec. 20, 1965, Ser. No. 515,232

, {5 Claims. (Cl. 148-121) This invention relates to low-carbon steelshaving improved drawability. More particularly, the invention isdirectedto a method of producing low-carbon sheet steel with improveddeep-drawing characteristics and high yield strength. Y It is well knownthat aluminum killed steels have excellent drawability; 'Aluminum killedsteels, referred to as,,fSK grade steel, are characterized by flattenedor paricake shap'ed ferrite grains which are crystallographicallyoriented to provide good drawability. Such grains are developed in thefinal sheet product by a properly controlled box annealing processduring which selective growth of'the favorably oriented grains iseffected by the. "aluminum 'nitride precipitate. Although the exactmechanism of the process is 'not fully known, the phenomenon that acritically dispersed second phase can markedly affect therecrystallization and grain growth is frequently observed, and in somecases, successfully employed in metallurgical applications.

However, aluminum-killed steels are relatively expensive due not only tothe cost of the alloy, but also to the low yield from ingots and highconditioning costs. It is, thereforefdesirable to develop a new methodfor producing deep-drawing sheets of relatively lessexpensive'low-carbon steels. R p

' The drawability'of'sheet material can be evaluated by simple tensiontests. When a strip specimen is pulled to a greater length; its widthand thickness are decreased. The plastie strain ratio 'can'vserve as anindication of the degree of mechanical anisotropy of the material. Thisratio is referredto as the R value and is defined as the ratio. ,ofpercent change in width (e the width strlainyto the. percent change inthickness (e the thickness strain), i.e. 5 V

t ne-raw Law the. width and length, respectively, of 'the, gaugesection, and the'subscripts i and 1 refer tolthe' initial and'final'measurements (before and after straining): of these dimensions. Thisexpression is based on the'fas'surnptibnthat the'volur'ne of the gaugesection re inainsfconstan't during testing and it eliminates thedirect'measurementjof the thickness which owing to its s'm'all' valu ein: a sheet material'yields less accurate resultsl'f The R 'value,"istherefore, a useful parameterfor indicatin'g thedegree of mechanicalanisotropy of a given materi alQForfan' isotropic sheet, the R value isequal to one. If is'les's'jthan" one, the sheet thins unduly and is,thereforeimdesirable for drawing operations. For deep drawing, it ispreferable to have R values equal to or gre ate-rj than about 11.5however, for some applications, materialwith lower R valuesmaybesatisfactory.

To obtain "an, 'ayerageR value, tensile tests are conducted: on.severaljspecirnenstaken at various angles, iisuallyfatid," 45fandf 90 tothe rolling direction. An average R value" of the sheet can then beobtained as follows:

Patented Oct. 1, 1968 'ice The difference among the individual R valuesindicates the earing tendency of the sheet in the drawing operation. Thelarger the difference, the stronger is the tendency for caring.

It has been found that the crystallographic orientation of the grains,and not the grain shape, is primarily responsible for the drawingproperties. We have discovered that the drawability and the R value canbe correlated with the crystallographic texture of the sheet. Gooddrawability and high R values are associated with the socalledcube-on-corner or the (111) texture, i.e. the (111) planes are parallelto the plane of the sheet. Poor drawability and low R values areassociated with the cube-on-face texture. The cube-on-edge or thetexture has intermediate drawing properties. Thus, for good drawingproperties, the amount of the (111) texture should be high, whereas thatof the (100) texture should be low. For a specific crystallographicplane, the R value varies also with the directions lying in the plane.Therefore, the ideal texture for optimum drawability is 111) fibertexture with the sheet plane normal as the fiber axis.

The crystallographic texture of a specimen is normally determined by theconstruction of complete pole figures from X-ray intensity measurements;however, for detection of small variation in the texture, a directcomparison of two pole figures cannot reveal the detailed differencesquantitatively. Accordingly, we have found it best to measure theintegrated peak intensities of several refiections from the plane of thesheet and express them in units of corresponding peak intensities of arandom specimen. The numerical values of these relative intensities soobtained are directly proportional to the pole densities of a specificplane lying parallel to the plane of the sheet. Since the drawability ofa sheet depends on the relative population of specific crystallographicplanes in the plane of the sheet, this technique is very useful. Theintensities of five different reflections, i.e. (110), (200), (112),(310) and (222) are measured. The intensity of the (222) reflectionwhich is the second order reflection of the (111) therefore representsthe amount of (111). texture. Similarly, the intensityof the (200)reflection represents the amount of the (100) texture, respectively.These two textures have, therefore, effects on drawability as do theircounterparts. The correlation between R values and texture has beenfound to be very consistent in actual test results.

We have found a method of producing low-carbon sheet steel of gooddrawability without sacrificing yield strength which involves acombination of steps applied to low-carbon steels having initiallygreater than 0.02% carbon. Our method produces a crystallographictexture with a high degree of (111) orientation and a lesser quantity of100) orientation. Thus, low-carbon steel processed according to theinvention has a very favorable average R value indicating gooddrawability. In addition, however, we have found that by controlling thefinal carbon content and precluding undue decarburization, a high yieldstrength may be maintained and a low-carbon steel having both improveddrawability and high strength can be produced. According to theinvention, a low carbon, hot rolled plate having more than about 0.02%carbon is cold rolled to from 50% to 85% reduction into sheet gauge. Thecold-rolled sheet is then annealed at normal annealing temperature inthe range from about 1025 F. to'about 1550 F. (below the transformationtemperature) for more than 10 hours in an atmosphere containing dryhydrogen. The term dry hydrogen as used herein refers tohydrogen havinga dew point less than 30 F. The sheet steel is annealed at the statedtemperature for the stated time until the carbon content is in the rangefrom about 0.004 to about 0.02% and then TABLE I.CHEMICAL COMPOSITION OFSTEELS C Mn Si S P A1501 N O It should be noted that compositions A andB had a desired carbon content above 0.02% whereas composi tion C had alower initial carbon content. Each of these steels was hot rolled in acommercial mill under the following conditions.

TABLE II.HOT ROLLING CONDITIONS Temperature, F. Finish Thick- Steel ness(in.)

Enter Finish Coil The microstructure of the hot-rolled plate from thesesteels consisted of equiaxed grains of about #8 ASTM grain sizes. Incompositions A and B, small colonies of fine pearlite were found toexist between the grains and at three grain junctions and carbide plateswere present at the grain boundaries. In composition C,pearliteicolonies were rare; however, a few thin carbide plates existedat some of the grain boundaries.

Crystallographic examinations (the results of which are reported inTable III below) of the hot rolled plate showed only minor differencesbetween the three steels, except that the (110) intensity of compositionC steel was very high and the (222) intensity was lower than in eitherof compositions A or B. The texture of compositions A and B werepractically the same.

TABLE III.RELATIVE INTENSITIES OF SELECTED X RAY REFLECTIONS FROMHOT-ROLLED PLATE, STEELS A, B, AND C A B C In aluminum-killed steels,crystallographic texture is controlled by precipitation of aluminumnitride. In lowcarbon steels treated according to the invention, thecrystallographic texture is controlled utilizing the cementite normallypresent. This may be accomplished by various annealing treatments suchas the solution and tempering treatments prior to cold rolling, ordecarburization during recrystallization anneal to eliect thedevelopment of desired textures. It'has been found that excellent Rvalues can be obtained in low-carbon steels having initial carboncontents of more than about 0.02%, such as compositions A and B, byproper annealing in a dry hydrogen atmosphere whereby the carbon contentis reduced to within the range of from 0.004 to 0.02% carbon. It hasalso been found, as will be shown hereinafter, that the treatment didnot produce satisfactory results for composition C because the initialcarbon content was already below the 0.02% limit.

Hot-rolled plate samples of compositions A, B and C were cold rolled to70% reduction in thickness from 0.096 to 0.029 inch for compositions ,Aand B, and from 0.086 to 0.026 for composition C. Thesarnples'weredhnannealed at a temperature of 1320 F. in dry hydrogen having a dew pointof approximately 90 F. The specimens were held at annealing temperaturefor 20 hours after which they were allowed to cool in the furnace. Thetexture of the samples together with that of a typical aluminum-killed(SK grade) deep-drawing steel are shown in Table-IV; TABLE Iv.-RELATIvEINTENs'ITIEs, OF SELECTED RAY REFLECTIONS FROM DRY HYDRO GEN ANNEAL-ELow-CARBON sTEELs AND A TYPICAL SK STEEL A I B C It can be seen that theunfavorable texture components, i.e. (200) and of compositions A and Bare sub; stantially lower than those of the SK grade, whereas the mostfavorable texture component (222) is higher than that of the SK grade.On the other hand, the (110) and (222) components of composition C areequal ,to" those of the SK grade, but the (200) and (310) components areappreciably higher. The (112) component of all three low-carbon steelsare lower than that of the SK steel.

The R values indicating relative drawability and yield strength andgrain size, as determined from duplicate samples of these steels whichwere also tested are shown in Table V. Corresponding data for the SKgrade steel are also listed for comparison. 7 TABLE V.THE R VALUES,YIELD STRENGTH, AND

GRAIN SIZE OF DRY HYDROGEN ANNEALED LOW- CARBoN sTEELs As COMPARED WITHTHOSE OF A TYPICAL SK STEEL v si; 5 1:73; 1 1.31 3 2.22; 1.64; 23.2 'asGrain size As can be seen, the R values of compositions A and B areequal or superior to those of the SKLgrad'e steel and the R values forcomposition C are notas satisfactory. The grains in all three carbonsteels, in contr'a'stt'o the SK grade steels, are equiaxed. I

The extent of decarburization dependsto some extent on'the flow rate ofthe hydrogen-containing atmosphere and the surface exposure of thespecimen. With'a' flow rate of approximately 6080 ccf/minute, the carboncontent may be reduced to as low as 0.004%, if care is exercised in theplacement of. the sheets. If the sheets are placed loosely in contactwith each other, the carbon content can be maintained at about0.016%,Extensive testing indicates that consistently good texture, high :Rvalues and high yield strength are. always obtained iflthe carboncontent in the annealed strip is reduced' jto below 0.02%. On the otherhand, we have found that if'the' ii'nal carbon content is greater than0.02%, the texture and the R values of the annealed strip,ar'einvariably poor,' The results of numerous tes'tsindicatethatduring'thei'i'di'y hydrogen anneal, the desired crystallographic textureis developed through the influence of the iron carbide p'recipitate onthe growth characteristics of the grains,

The effect of heating rate and soaking tirn efon" plastic strain ratiois shown in'Tablc vL 'These results showftliat at a constant soakingtime theave'ra'ge'R yalues decrease slightly with increasing heatingrate and with' a constant heating rate, R decreases with decreasingfs'oak i ng time". The remaining carbon content correlates withtheRvalues and shows that poor R values 'are associated "with high carboncontent, i.e. carbon contents g'reater thari0.02% The remaining carbon,contents indicate also that the effective decarburization occurs mainlyduring the soaking period of the annealing treatment.

TABLE VL-EFFECT OF HEATING RATE OR SOAKING TIME ON THE AVERAGE PLASTICSTRAIN RATIO, R, AND THE REMAINING CARBON CONTENTS Heating r ate Soakingtime 0, percent (time to 1,320" F.) (Hr. at 1,320 F.)

A B A B To illustrate the importance of using dry hydrogen, a series oftests were conducted which differed only in that in some dry hydrogenwas employed and in others wet hydrogen was used. Although, as is known,wet hydrogen is more effective for decarburization than dry hydrogen, wehave found that excessive decarburization and grain growth results withthe use of wet hydrogen even though acceptable R values may be obtained.Table VII reports the results of four tests of compositions A and B, twoof which were conducted with the annealing performed in dry hydrogen andtwo with the annealing in wet hydrogen. The results of these tests arereproduced in Table VII.

TABLE VII Dry Hydrogen Wet Hydrogen A B A B R 1. 99 1. 59 1. 73 1. Y.S.(K psi.) 17. 8 23. 2 10.0 9. 8 Grain size (ASTM N 0.) 6. 0 7. 0 6-1 7. 0Carbon Content 0. 007 0. 004 0015 0015. 0014 As can be seen, thenitrogen content has been lowered significantly. Since unfixed nitrogenis a principal cause of strain aging of low-carbon steels, theundesirable effects of agingon mechanical properties can be minmized bythis treatment.

TABLE VIIL-CHEMICAL COMPOSITION OF STEEL AFTER DRY HYDROGEN ANNEAL SteelC Si S The strain aging index of dry hydrogen annealed products is shownbelow in Table IX.

TABLE IX.- Strain aging index of dry hydrogen annealed steels(prestrained 8%, aged 4 hrs. at 212 F., and retested) Percent increasein yield stress (Strain Aging Index):

The percent increase in yield stress when the specimen is prestrained 5%minimum or beyond discontinuous yielding, aged 4 hours at 212 F., thenretested.

The above values compare favorably to the strain aging index ofconventional box-annealed rimmed steels, which usually ranges between 20to 25% It is apparent from the above that certain changes andmodifications may be made without departing from the invention. It isseen, however, that it is essential for the successful practiceaccording to the invention for the inital carbon content of the steelstreated to be above 0.02% and, further, that the carbon be reducedduring the dry hydrogen anneal to from between 0.016 to 0.004%. Theeffect of this treatment is to impart improved drawability withoutsacrificing yield strength. It is also shown that too extensivedecarburization results in excessive grain growth which impairs thedrawability and yield strength of the sheet steel. Moreover, the productis relatively nonaging as Well as deep drawable due to the fact that thedry hydrogen anneal also reduces the nitrogen content considerably,i.e., to about 0.001%.

We claim:

1. A method of producing low-carbon sheet steel of improved drawabilityand high yield strength which comprises cold rolling hot-rolled plate oflowcarbon steel having more than 0.02% carbon to from 50 to 85%reduction into sheet gauge, annealing said cold-rolled sheet at atemperature in the range of from about 1025 F. to about 1550 F. for morethan ten hours in an atmosphere containing dry hydrogen having a dewpoint less than 30 F. to result in a carbon content of from 0.004 toless than 0.02% and cooling the annealed sheet.

2. A method according to claim 1 wherein said hotrolled sheet is coldrolled to from to reduction.

3. A method according to claim 1 wherein said coldrolled sheet isannealed at a temperature in the range of 1200 F. to 1400" F.

4. A method according to claim 3 wherein said coldrolled sheet isannealed for more than 15 hours.

5. A method according to claim 4 wherein said coldrolled sheet isannealed for about 20 hours.

References Cited UNITED STATES PATENTS 2,360,868 10/ 1944 Gensamer148-16 3,239,388 3/1966 Sasaki 148-12.1 3,239,389 3/1966 Yoshida148--12.1 3,244,565 4/1966 Mayer et al 14812.1 3,262,821 7/1966 Yoshida14812.1 3,281,286 10/1966 Shimizu et a1. 14816 3,348,980 10/1967Enrietto l48-l2.1

OTHER REFERENCES Low et al., vol. 158, AIME Transactions, Iron and SteelDivision, p. 209 et seq.

HYLAND B IZOT, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

1. A METHOD OF PRODUCING LOW-CARBON SHEET STEEL OF IMPROVED DRAWABILITYAND HIGH YIELD STRENGTH WHICH COMPRISES COLD ROLLING HOT-ROLLED PLATE OFLOW-CARBON STEEL HAVING MORE THAN 0.02% CARBON TO FROM 50 TO 85%REDUCTION INTO SHEET GAUGE, ANNEALING SAID COLD-ROLLED SHEET AT ATEMPERATURE IN THE RANGE OF FROM ABOUT 1025*F. TO ABOUT 1550*F. FOR MORETHAN TEN HOURS IN AN ATMOSPHERE CONTANING DRY HYDROGEN HAVING A DEWPOINT LESS THAN -30*F. TO RESULT IN A CARBON CONTENT OF FROM 0.004 TOLESS THAN 0.02% AND COOLING THE ANNEALED SHEET.